This invention relates to communications antennas used to enhance two-way communications, and particularly to those in the 27 megahertz citizens band (commonly referred to as CB antennas). They are especially useful in base station installations.
Radio waves are electromagnetic waves which are made up of electric and magnetic fields which are perpendicular to each other. These fields, in turn, are propagated in a direction perpendicular to the plane in which the fields exist. The electric field exists in the same plane that the electric conductor lies. If the conductor is perpendicular to the earth, the electric field therefore is vertical to the earth and the conductor is said to be vertically polarized. Similarly, if the conductor lies in a plane horizontal to a plane on the earth's surface, it will propagate its electric field in this plane and the conductor is said to be horizontally polarized.
It is necessary that the radiating conductor or antenna used for receiving a signal is polarized in the same plane as the plane in which the transmitting antenna propagates its signal. If the transmitting and receiving antennas are not so polarized, a pronounced loss of signal strength is experienced, often by as much as 15 decibels.
Ideally, it is desirable to use a directional or beam antenna to concentrate the available transmitted power in its "pointed" direction, i.e., the direction of the receiving station. However, if the operator is being called by a station lying in a direction outside of a beam antenna's sector of azimuth, the station might not be heard. To be heard, it is necessary for the operator to rotate the beam antenna, which must therefore be provided with a suitable rotary turning mechanism to attempt to pick up a station's call. By the time this has been done, the calling station may have stopped transmitting. Accordingly, both in CB and amateur radio communications, because most communications are from remote stations at unknown directions of azimuth, it is desirable to utilize omnidirectional receiving antennas so that signals from all azimuths are readily received, albeit at lower signal strengths than would be derived from beam antennas. Of course, an omnidirectional antenna may be used to receive a signal to "home" in on a transmitting station, following which a beam antenna may be used to obtain maximum signal strength.
Most beam antennas in current use in the high frequency spectrum, ranging from about 3 to about 30 megahertz, are horizontally polarized. Because the structures generally used to effect horizontal polarization in the omnidirectional mode are too large, the use of a separate omni antenna for establishing contact initially, following which the beam antenna is switched to, has not been as practical or as feasible as has been the use of vertical omni and vertical beam antennas.
Horizontally polarized beams are popular because they are relatively immune to picking up made-made noise that is randomly present in a vertically polarized mode. Vertically polarized antennas used for receiving suffer in this respect. Accordingly, the provision of a horizontally polarized omni antenna is highly desirable.
Further, because each station licensed for operation in CB and other amateur services operates on unassigned frequencies or channels within a defined spectrum of a respective narrow band, interference from other stations is often great. The introduction into the CB and amateur fields of a beam antenna by the assignee of the present application about eight years ago which made provision for optionally switching from horizontal to vertical polarity resulted in a dramatic change in the antenna selecting attitudes of, and possibilities available to the communicator. The success of that antenna was due to the ability of a station operator to switch from the vertical to the horizontal mode, if the other station antenna was similarly polarized or polarizable, when interference was rampant in a vertical mode. The provision now of an effective horizontally polarized omni antenna makes it possible to assist the communicator in homing in on a distant station and to use the preferred, and usually less crowded, horizontal polarization.
In the very popular citizens' band (CB) radio service, the assigned frequencies are in the eleven meter band (26.965 MH-27.405 MH). It requires no elaborate explanation to understand why vertically polarized antennas have been used. The difficulties of supporting a horizontal mobile antenna from vehicles such as automobiles where one-half the wave length is approximately 18 feet, are obvious. Indeed, the use of horizontal antennas even at base stations where antenna elements must be in the order of 18 feet in length is difficult.
While the simple vertical antenna inherently produces omnidirectional radiation, a simple antenna, such as a dipole disposed horizontally does not produce usable omnidirectional radiation. It produces bidirectional radiation instead. Omni-directionality of horizontally polarized waves is difficult to obtain easily and requires either more than a single antenna element or some supplemental technique, such as the rotation of an antenna element.
One approach to obtain horizontally polarized omnidirectional radiation would be to modify the simple dipole. For example, it is possible to fold the ends of the dipole to form a loop with the ends of the dipole spaced slightly apart. Such antennas have been constructed and are known as "halo" antennas. Such antennas do have some significant deficiencies, however. For example, the bandwidth becomes relatively narrow as a result of the amount of shortening required for omnidirectional operation. In addition, the gain of such antennas also appears to be reduced.
Another approach that has been taken is the clover leaf or "big wheel" multi-element antenna. In these structures, each of the leaf elements is a full wave length element, bent to form a half wave circumferential radiating segment with quarter-wave radial feeders, with each of the half-wave segments being end fed in phase. Unfortunately, these configurations are limited in size and, therefore, are used for higher frequencies because of structural limitations. Thus, the use of multiple or combined antenna elements to produce horizontally polarized radiation has presented significant physical and mechanical problems in the area of citizens' band frequencies.
In the eleven meter CB band, the approximate length of a half-wave length dipole antenna is equal to approximately eighteen feet. It therefore becomes clear that the construction of omnidirectional horizontally polarized antennas in the CB band presents significant structural problems directly related to the size of the antenna elements required in this frequency range. Further, while in some environments the utilization of a complex of separate antennas physically oriented in different directions to effectively provide the desired omnidirectionality may be possible, in most situations an antenna must be as compact as possible, should be physically integrated and should utilize minimal space. In most instances, it should be supported on a single supporting mast. These conditions particularly apply to CB antennas which are often installed in residential areas where the available space is restricted.